Ind. Eng. Chem. Res. 1996, 35, 4111-4118
4111
Chemical Processing in High-Pressure Aqueous Environments. 5. New Processing Concepts L. John Sealock, Jr., Douglas C. Elliott,* Eddie G. Baker, Alexander G. Fassbender, and Laura J. Silva Pacific Northwest National Laboratory,† P.O. Box 999, Richland, Washington 99352
A combination of fundamental and applied process research at Pacific Northwest National Laboratory resulted in the development of several new processing concepts. These concepts have in common the use of high-temperature pressurized water as a unique reaction medium for carrying out chemical reactions. Typical operating conditions of these processes range from 300 to 360 °C and have pressures to above 200 bar. Investigation of the process chemistry and engineering for these new processing concepts required the development and scale-up of several high-pressure reactor systems which are described. Several new processes involving catalysts and the distinctive properties of pressurized water were patented as part of this effort. These processes, which are at various stages of development, address a broad mix of energy and environmentally related problems. Chemical processing in a high-temperature liquid water environment remains a relatively untapped area for commercial application. Introduction
Table 1. Summary of Processing Concepts
Researchers at the Pacific Northwest National Laboratory (PNNL) developed several new processing concepts using high-temperature pressurized water as a unique reaction medium. This research focused on the discovery and development of new chemical processing systems to provide progressive alternatives to current technologies. The fundamental and applied processing background and historical perspective of this work which provide the underpinnings of these new concepts have been described previously in this journal (Sealock et al., 1993). The objective of this paper is to provide a technical overview of the chemical processes that are being developed at PNNL which employ pressurized water as a reaction medium. Processing objectives and operating conditions, which are summarized in Table 1, are presented for each technology, and references are listed where additional information regarding the specific processes can be found. Background Research at PNNL is addressing the use of hightemperature pressurized water as a reaction medium for hydrothermal processing. This work resulted in the discovery and patenting of several new processing concepts. These processes address a broad mix of energy and environmentally related problems including energy production from wet biomass and food processing waste; waste minimization and waste reduction; destruction of hazardous organics in industrial wastewaters and groundwater; oil recovery and hazard reduction related to oil processing wastewater disposal; and improved chemical routes to energy and chemicals. All of these concepts have in common the use of highpressure, liquid water systems operating at belowcritical conditions. Typical operating conditions range from about 300 to 360 °C and have pressures to over 200 bar. † Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76-RLO 1830.
S0888-5885(96)00119-4 CCC: $12.00
operating param process
purpose
temp, °C
pressure, bar
TEES NitRem STORS ABCS PST
gasification, organic destruction nitrogen conversion sludge liquefaction water-gas shift reaction break oil/water emulsions
250-350 350-360 265-350 200-350 300-350
150-200 200-240 a 40-200 400-200
a
Pressure should be high enough to prevent vaporization.
The breadth of the research spanned the full range of process development activities including fundamental investigations, bench-scale development, pilot-scale operation, and commerical design. The technical background and underpinnings of these processes (Sealock et al., 1993) provided insight into the use of pressurized water as a reaction medium and information on the effects of temperature, pressure, catalysts, and feedstock variables on processing. Other important processing considerations are addressed in this paper, including reactor system development, process scale-up, and applications. Reactor System Development and Scale-up Issues In order to investigate chemical conversion and reactions in high-temperature pressurized water, several reactor systems were built and operated. Each reactor system was designed to be flexible with respect to obtaining analytical samples, processing data, and process scale-up information. Research performed in these reactors evolved through four distinct levels of reactor system development which have been described separately in the literature and are summarized in Table 2. These reactor systems include a batch reactor; a continuous, stirred-tank reactor (CSTR); bench-scale fixed-bed tubular reactors; and pilot-scale tubular reactor systems. These reactor systems were critical to obtaining fundamental conversion, product distribution, and processing-related information necessary to understand the chemical reactions under these conditions and to develop new processing concepts. Some of the important © 1996 American Chemical Society
4112 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 Table 2. Reactor Systems for Process Development size
material
operation
1 L autoclave 1 L Carberry CSTR 1 ft long × 1/2 in. i.d. tubular reactor 6 ft long × 1 in. i.d. tubular reactor (2) 6 ft long × 2 in. i.d. tubular reactors 10 gal/h dual-shell reactor
Inconel Inconel 316 SS 304 SS 304 SS Cr-Mo/Ti
batch screening tests kinetic studies continuous catalyst life tests continuous feedstock tests continuous pilot-scale operation continuous pilot-scale operation
Figure 1. Dual-shell pressure-balanced reactor vessel drawing.
information obtained using these systems included feedstock performance and comparison studies; evaluation of temperature and pressure effects; catalyst screening, performance, and lifetime studies; conversion and kinetic studies; and equipment design and operability. A key aspect of the research in these reactor systems was the special attention given to safety design considerations. Each system design was based on ASME codes for vessels and the best available information on pressure ratings of components. Appropriate quality assurance concerning pressure vessel certification and operation was incorporated into written Safe Operating Procedures. Safety features built into the equipment designs included fail-safe valves to prevent overpressure situations in the case of control system failure, manual depressurizing capability, and use of relief valves and vented rupture disks. Barricades were built around critical components to further shield operators in the event of system failure. A required, specialized safety training session for all operators was equally important in ensuring operator safety. In addition to the development of new processing concepts to be discussed in this paper, equipment safety concerns spawned a new reactor design technology (Fassbender, 1992) for existing wet-air oxidation and supercritical water oxidation processes. An engineering-scale (10 gal/h) dual-shell reactor was designed and built for destruction of hazardous organic components in waste water. Following is a description of this reactor technology. Dual-Shell Vessel. The dual-shell reactor design (Figure 1) is specifically applicable for safe and economical processing in the corrosive environments that occur in wet-air oxidation or supercritical-water oxidation systems. The dual-shell design is based on the concept of a separate inner shell insert that fits a few millimeters from the inner wall of the pressure vessel. Syltherm 800, a high-temperature heat-transfer fluid, is placed in the annulus between the pressure vessel and the insert. The Syltherm 800 is kept physically separate and in hydrostatic equilibrium with the process fluid entering the reactor. The pressure balancing is achieved by an external piston which permits fluid volume change to respond to pressure and thermal forces. The insert is typically constructed of grade 9 titanium, and the pressure shell is constructed of chrome-moly steel. The thickness of the titanium insert is determined by corrosion potential rather than strength;
thus, it is thick enough to support its own weight and act as an envelope to separate the corrosive reactants from the steel pressure vessel. The design provides adequate space between the insert and the pressure vessel to allow for differential thermal expansion. The dual-shell design eliminates the untenable choices between strength and corrosion resistance, temperature, pressure, and vessel life; safety vs cost-effective operation; and maintenance. It eliminates the constraint of having to carefully match thermal expansion coefficients of dissimilar metals. This elegant design combines ordinary steel and specialty metals to meet the challenges of corrosive chemistry in pressurized processes and allows designers to select the optimum materials for corrosion resistance separately from pressure vessel considerations. Thus, the strength and corrosion resistance specifications of the pressure vessel are decoupled from the specifications of the insert. As new materials are developed, they can be quickly and easily incorporated into existing dual-shell reactors. Maintenance is greatly simplified since insert replacement can be accomplished without moving or modifying the pressure vessel. The 10-fold cost savings in materials, maintenance, fabrication, and certification realized by the dualshell design greatly improves the economic viability of hydrothermal waste destruction. Perhaps more important than the economic cost savings are the safety advantages. The dual-shell design literally provides two added measures of safety by continuously monitoring the integrity of the insert, thereby assuring the pressure vessel is not exposed to a corrosive environment. Conductivity probes measure the electrical resistance of the pressure-transfer fluid in the pressure vessel annulus. A separate position indicator measures and monitors the position of the hydraulic balancing piston. Significant or unusual changes in either the conductivity of the oil or the position of the balancing piston signify a breach of the insert or a leak in the pressure balancing hydraulic line and alert computer control to shut down the system and relieve the pressure. Maintenance operators can then replace the insert or correct the problem. The steel pressure vessel is not subjected to corrosion. The outer pressure vessel is a permanent asset, but the insert may be expendable. The reactor is designed with a full aperture end closure and provisions for preventative maintenance, repair, and replacement are part of the design for full-scale operation. Built-in removal and replacement features mean less downtime and easy recovery and recycling of expended inserts. The pressure balancing oil is also used as a heat-transfer oil to regulate the process temperature. Getting heat in and out of the reactor is achieved with greater efficiency due to the thin wall of the insert compared to the pressure vessel. The design, welding, quality assurance, and operation of steel pressure vessels are well-known and codified by the ASME pressure vessel code and state inspection authorities. Carbon steel and chrome-moly steel pres-
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4113
Figure 2. TEES (Thermochemical Environmental Energy System) process schematic.
sure vessels are relatively inexpensive and easy to fabricate, test, and certify. The dual-shell reactor design is licensed to Innotek Corp., Little Rock, AR. An engineering-scale reactor was designed, built, and tested for hydrothermal destruction of munitions waste. New Process Systems Several new processes for carrying out gasification, organic and nitrate destruction, liquefaction, gas processing, and separations have been developed. A description of each processing technology is given below based on the specific application of the process. Gasification/Organic Destruction. (a) TEES. The Thermochemical Environmental Energy System (TEES, a registered trademark of Onsite*Ofsite, Inc., Duarte, CA) is a catalytic gasification process (Figure 2) developed to produce fuel gas from lignocellulosic feedstock (Sealock et al., 1988). The process has broad application to many feedstocks ranging from highmoisture biomass crops to waste products from food processing operations. Conversion of the high-moisturecontaining feedstock is carried out in an aqueous environment with no need for drying or dewatering. A combination of metal catalyst and alkali is employed to produce a methane-rich, medium-Btu fuel gas and clean water. Maintaining a liquid-phase reaction environment through the use of a reduced-temperature (350 °C), pressurized water facilitates more efficient heat recovery from reaction products, which, in turn, allows the use of very high-moisture feedstock relative to conventional approaches to biomass gasification. Pressures of about 150-200 bar are used in the system to prevent
vaporization of the water. The resistance to heat transfer in conventional heat-transfer equipment is much lower with liquid water than with supercritical water or steam; thus, the required area for heat transfer will be less. At these conditions, heat from the hot product steam can be economically recovered for heating the feed stream in compact heat exchange equipment. High reaction rates, even at the relatively low temperatures employed in the process (350 °C), are achieved by the use of reduced nickel metal catalysts. The reaction conditions serve to shift reaction equilibrium toward the production of methane, as opposed to lower valued fuel gases such as carbon monoxide and hydrogen. Residence time requirements for many feedstocks are estimated to be less than 10 min, which compares favorably with the requirements in conventional thermal systems operating at much higher temperatures, typically above 600 °C (Baker et al., 1989; Elliott and Sealock, 1985). The process is a flexible technology designed for the treatment of a wide variety of feedstocks ranging from dilute organic in wastewater to more concentrated waste sludges. Examples of the wide variety of biomass and organic chemicals that have been gasified using TEES are shown in Table 3. Wet biomass feedstocks that have been processed in the bench-scale tubular reactor system include spent grain (4 wt % dry solids), rum vinasse (5 wt % dry solids), and potato crumbs (14 wt % dry solids) (Elliott et al., 1995). Greater than 98% reduction of chemical oxidation demand (COD) was achieved for the spent grain at liquid hourly space velocities up to 3.5 h-1. COD conversions of vinasse and potato crumbs ranged from 77% to more than 97% at liquid hourly space velocities greater than 2 h-1. The product fuel gas composition includes ranges from 45% to 60% methane; ranges from
4114 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 Table 3. TEES Feedstocks Tested gasification
destruction
herbaceous
aquatic
processed wastes
real waste streams
model compounds
napier grass sorghum sunflower corn stover
hyacinth kelp
potato waste spent grain grape pomace cranberry pomace anaerobic disgestion sludge kraft black liquor cheese whey coffee grounds spent grain liquor vinegar olive water chicken processing waste fish processing waste gelatin manufacturing waste wood gasification wastewater rum vinasse soda pulp wastewater potato product crumbs shrimp waste potato peels diary waste onions corn canning DAF apple pomace tar hydrotreating wastewater
nylon wastewater acrylonitrile wastewater fatty acid waste metal chelate solution sodium cyanide waste polyol wastewater vitamin fermentation broth PO/SM wastewater paint booth wash
methyl ethyl ketone methyl isobutyl ketone ethanol propylene glycol acetic acid phenol cresols (o,m,p) sodium phenolate sodium benzoate benzene anthracene biphenyl polystyrene nitrobenzene nitropentane sodium cyanide mixed nitrophenols hexane pentadecane eicosane chloroform carbon tetrachloride chlorobenzene trichloroethylene aniline pyridine benzenesulfonic acid pentanesulfonic acid
35% to 50% carbon dioxide;
